There are many methods provided for shielding electromagnetic interference (EMI). However, shields made from metal are cumbersome, heavy and complex in forms to thereby be unsuitable for electronic industries. The metal coating on the surface of plastic article for making EMI shield may be easily scratched to partially lose its shielding efficiency. Once the scratched scraps, which are electrically conductive, drop onto a printed circuit board of an electronic product, a short-circuiting may be caused to deteriorate the electronic product. Meanwhile, the metal coating or plating may increase the problems of environmental protection.
Recently, several plastic molding processes were disclosed by incorporating metal materials into the resin compositions for making EMI shields.
U.S. Pat. No. 4,474,685 entitled "High Performance Molding Compounds for Shielding Electromagnetic Interference" to Myron C. Annis discloses a molding composition comprising a thermosetting resin binder and an electrically conductive filler comprising particles of carbon black, graphite and a conductive metal for achieving a shielding effect to the emissions of electromagnetic interference.
However, when blending the resin with the particulate fillers for molding processing for making EMI shields, the particulate fillers may be easily clustered to cause unhomogeneous dispersion of the electrically conductive fillers in the resin matrix, thereby influencing the shielding effectiveness of the molded products.
Other fillers such as fiber fillers and flake fillers may also be considered to substitute the particulate fillers as abovementioned.
However, the process by using fiber fillers is costly and uneconomic for a commercial production.
The flake fillers, when used in the processing steps of resin blending, pelleting and injection molding, may be easily broken to reduce the electrical conductivity, thereby possibly decreasing the EMI shielding effect of the molded product.
The present inventors have found the drawbacks of the conventional methods for making EMI shields and invented this process for making homogeneously metallized plastic pellets for molding effective EMI shields.